The present invention relates to the detection of targets in a scene with a ranging device or ranging process such as radar. More specifically, the invention relates to the detection of targets entering, leaving, or moving in a scene containing background clutter such as roadbeds, telephone poles, and railroad tracks even though the target does not return a significant amount of energy over and above the energy returned by the background clutter.
Radar, an acronym for Radio Detection and Ranging, is used to detect, determine range, measure angles, and determine velocity of targets. Radar is one in a class of ranging devices including, for example, sonar. In operation, a radar typically generates high voltage pulses, sends them through a radio frequency amplifier, then applies the amplified pulses to an antenna for radiation into space or into a scene. The antenna may be steered, possibly by a servo-driven pedestal, or it may be stationary. A stationary radar may also be referred to as a "staring" radar because it always radiates pulses, i.e., "looks", at the same scene.
The radiated pulses coming off the antenna travel through space until they completely dissipate, or until they hit an object or a target. Objects and targets reflect the radiated pulses back to a receiver. The receiver is connected to additional electronics, an amplifier, typically, and additional signal processing equipment that extracts information from the received pulses and displays that information on a screen or provides the information to subsequent processing electronics such as a filter. The received pulses may be converted to digital samples during this process. The process of sending a radiated pulse and gathering the received pulse is often referred to as a "sweep" of the background.
There are no fundamental limits of the frequency of radiated pulses that a radar may use. Radars have been operated at wavelengths spanning many orders of magnitude, for example, from 10.sup.-7 m to 100 m. Furthermore, there is no requirement that a radar use an antenna per se. Rather, the radar need only provide an emitter that sends radiated pulses into a scene and a receiver that gathers the received pulses reflected back. An alternative emitter may be chosen by taking into account a number of factors including the wavelength of the radiated pulses. For example, for radiated pulses that have a wavelength in the visible spectrum, the emitter may consist of a light emitting diode rather than an antenna. The receiver may then consist of a photodiode or optocoupler. This flexibility makes radar useful in many diverse applications including air surveillance, weather forecasting, and astronomy.
Because a staring radar always looks at the same scene, it may be used to monitor a border, for example to detect people trying to enter a compound or cross a boundary or perimeter. Such a radar could also be set up to look down a set of railroad tracks to detect movement along or across the tracks, detect targets in the field, and alert someone monitoring the area. One way to process data in a ranging application is to divide the radar's range into discrete segments. A discrete segment is termed a range "bin" and may, for example, be approximately one foot long. The signal processing equipment in the radar would then take the received pulses and return a range profile that represents the energy in the received pulses as a function of range. The range profile may consist, for example, of a vector (a single column of numbers) which represents the amplitudes of the energy returned in the received pulses for each range bin. A typical target, for example a person, may be wide enough to cover two or three range bins. Some targets are difficult to detect, however, due to background clutter present in the scene.
A staring radar does not always look out into empty space. In most instances, the radar looks at a scene that may include large amounts of background clutter. Background clutter generally refers to everything except the target that the radar is trying to detect and includes, for example, telephone poles, railroad rails, roadbeds and other fixed objects in the view of the radar. A staring radar may, for example, have a range of 1000 feet. The background clutter would then include all the fixed objects in the entire 1000 foot range.
Previous radars have had difficulty picking out targets from among the background clutter because the background clutter, although fixed, returns different amounts of radiated energy in the form of received pulses from one sweep to the next. In other words, the background clutter does not present the same amount of returned energy from one sweep to the next. Rather, the background clutter returns energy in received pulses that exhibit a high variance around the average value of the energy in the received pulses. Thus, although the average energy in a received pulse may stay approximately the same over a long period of time, the variance in amount of energy in a received pulse is substantial. The variance is caused, in part, by preexisting electromagnetic noise in the scene, multiple return paths for the received pulses, and noise in the radar system electronics. The variance prevents previous radars from determining when, exactly, a desired target is in the scene and a detection should be declared.
These previous radar systems, expect each sweep of the background clutter to return the same average energy, and would identify (incorrectly) a disturbance above the average (caused by variance in received pulses around the average) as a detection. Furthermore, if the radar takes into account the variance in the received pulses, then the radar is unable to detect small targets, or targets returning small amounts of energy. The inability of the radar to detect small targets is due in part to the fact that the average energy of the received pulses in a scene is much larger (due to the large objects composing the background clutter) than the energy returned by a small target. Because the average energy is large and the variance in received pulses is a significant fraction of the average energy, a typical radar cannot detect small targets.
The additional energy that the small targets return in a received pulse is not much larger than the variance in background clutter from one sweep to the next. In fact, the additional energy may be far smaller in many cases. Therefore, the radar, without taking into account additional considerations, cannot determine if the additional energy is, in reality, a new target entering the scene, or simply variance in the received pulses returned by the background clutter.
In the past, radar systems have addressed the problem of variance in the received pulses using a Constant False Alarm Rate (CFAR) technique. CFAR recognizes that there will always be a given amount of variance in the energy content of the received pulses, including variance due to other factors including the radar's own internal noise, for example, thermal noise. In order to avoid numerous false alarms due to variance in the received pulses, CFAR chooses a limit, called the constant false alarm rate. The energy content in a receive pulse must lie above the CFAR limit before the radar detects a desired target. The CFAR limit is based on an analysis of the probability density function of the received pulse variance. A value is selected from the probability density function that gives an acceptable percentage of false alarms. Regardless of the CFAR limit, however, a false alarm may still occur.
It may be assumed that the noise causing the variance in received pulses is a Gaussian function (i.e., the probability density function of the variance has tails that go out to infinity), and thus there will always be a received pulse with enough variance to cause the received pulse energy level to rise above the CFAR limit. Hence, it is always possible to detect a target when no target, in reality, is present (i.e., on the basis of noise only). This false detection is called a false alarm. Choosing the CFAR limit thus sets the false alarm rate. The CFAR limit may be chosen, for example, such that false alarms occur 1 in 1,000 sweeps or 1 in 10,000 sweeps. A higher CFAR limit reduces the false alarm rate, but also increases the size of the smallest target the radar can detect.
Thus, previous radars operate according to methods that have significant drawbacks. Either the radar ignores targets that do not return significant amounts of energy (including small targets) because the returned energy is under the CFAR limit, or the radar operates without a CFAR limit and therefore experiences numerous false alarms.
Thus, a need remains for an improved target detector which overcomes the disadvantages discussed above and previously experienced.